This application relates generally to communication systems, and, more particularly, to wireless communication systems.
Wireless communication systems typically deploy numerous base stations (or other types of wireless access points such as eNodeBs) for providing wireless connectivity to user equipment such as mobile units or other wireless-enabled devices. Each base station is responsible for providing wireless connectivity to the user equipment located in a particular cell or sector served by the base station. The air interface between the base station and the user equipment supports downlink (or forward link) channels for carrying information from the base station to the user equipment and uplink (or reverse link) channels for carrying information from the user equipment to the base station. The uplink and/or downlink channels typically include data channels for carrying data traffic such as voice information and control channels for carrying control signal such as pilot signals, synchronization signals, acknowledgment signals, and the like.
Multiple-input-multiple-output (MIMO) techniques may be employed when the base station and, optionally, the user equipment include multiple antennas. For example, a base station that includes multiple antennas can transmit multiple independent and distinct signals to multiple user equipment concurrently and on the same frequency band. MIMO techniques are capable of increasing the spectral efficiency (e.g., the number of bits/second/Hertz) of the wireless communication system roughly in proportion to the number of antennas available at the base station. However, the base station also requires information about the state of the downlink channel(s) to each of the user equipment to select user equipment that have approximately orthogonal downlink channels for concurrent transmission. The channel feedback may be provided by the user equipment on the reverse link, but this increases overhead associated with the MIMO transmissions, which reduces the spectral efficiency of the wireless communication system.